In recent years, a superconducting technology has been remarkably developed, and various types of examination apparatus such as a Magnetic Resonance Imaging (MRI) apparatus and a Nuclear Magnetic Resonance (NMR) apparatus have been used. In order to use the superconducting technology, a cryogenic temperature of 10 K or less, and further, 4 K or less is required to be realized. In order to realize such a cryogenic temperature, a refrigerator called as a cold head is used.
There are various types of cold head such as one of Gifford-McMahon type (GM type), one of Stirling type, and one of pulse type. In any one of the types of cold head, to achieve the cryogenic temperature, a regenerator material is filled in a regenerator container called as a stage.
Depending on a design of the cold head, the stage is sometimes formed of one stage, and is sometimes divided into a plurality of stages such as two stages. Helium gas is passed through the stage, and a specific heat per volume of the regenerator material is utilized to obtain the cryogenic temperature.
One example of a cold head for obtaining the cryogenic temperature of 4K or less has a three-layer structure including a lead regenerator material, a HoCu2 regenerator material, and a GOS regenerator material (gadolinium oxysulfide regenerator material) filled in a second stage (second-stage regenerator container).
The above-described cold head can obtain the cryogenic temperature by causing an adiabatic expansion of refrigerant gas such as helium gas. As described above, in the cold head for obtaining the cryogenic temperature, a plurality of types of regenerator materials are filled in layers in the stage.
When the plurality of types of regenerator materials are used, since a specific heat peak temperature of each of the materials is different, it is not possible to mix and use the regenerator materials. Accordingly, when the plurality of types of regenerator materials are used, the material layers are partitioned by a metal mesh material.
As the regenerator material, for example, a regenerator material particle group with aligned particle size in which a proportion of particle having a particle size of not less than 0.01 mm nor more than 3 mm is 90% or more, and a proportion of particle having an aspect ratio of 5 or less is 90% or more, is used.
When the metal mesh material is used to partition the plurality of types of regenerator material particle groups, a problem in which mesh holes of the metal mesh material are clogged with the regenerator material particles, has arisen. If the mesh holes are clogged with the regenerator material particles, a permeability of refrigerant gas is lowered.
In the regenerator operation using the regenerator material particles, the adiabatic expansion due to a specific heat per volume of the regenerator material particles is utilized, so that it is preferable to fill as many regenerator material particles as possible in the stage. For this reason, the regenerator material particle groups are filled so as to be closely contacted to the metal mesh material. In addition to that, the permeability of the refrigerant gas has to be secured.